U.S. Drug Enforcement Administration agents discovered the malnourished animals, many injured and some standing in more than a foot of waste, while serving drug-related warrants on Tuesday at the 99-acre property in Graham, Washington, Pierce County Animal Control supervisor Brian Boman told Reuters.

Animal control officers and sheriff's deputies from Pierce and Kitsap counties returned to the ranch a day later to seize the animals and found many were highly skittish because they had been "stall-bound" in three dark barns, Boman said.

"It was like a puppy mill, only with horses," Boman told Reuters. "The conditions are terrible. There's no telling how long it's been since they've seen daylight."

Pierce County auditor Julie Anderson told NBC station KING 5 in Seattle that the horses had not been handled in a very long time. "They literally have their 'night eyes' on so they're very sun sensitive and are having trouble with depth perception," she said, describing the scene as "wanton criminal neglect."

The horses were receiving veterinary care and were being held for the time being as evidence, KING 5 reported.

No-one has been arrested so far but the owners could face charges of animal cruelty in the second degree, a gross misdemeanor in the state of Washington.

KING 5 reported that an attorney for the owner said his client "loves these animals" and did not believe the conditions reached a criminal level.

'No lighting or ventilation'Authorities spent more than nine hours rounding up the horses, which included mostly purebred Arabians as well as Belgian Draft horses and Clydesdales, to take them to nearby fairgrounds. None were race horses.

Some likely would be euthanized, Boman said.

A Pierce County Sheriff's Office news release, describing the roundup as the largest horse seizure the county had ever undertaken, cited the horses' living conditions as deplorable.

"Most of the horses were in barns that had large amounts of urine and feces in the stalls," the release said. "Some of the barns had no lighting or ventilation and the smell of ammonia was very strong."

Because federal and county criminal investigations are ongoing, federal authorities would not immediately release the name of the farm's owner, said Emily Langlie, spokeswoman for the U.S. Attorney's Office in Seattle.